The invention is directed to a salt bath for nitriding components made of steel and iron, the bath consisting essentially of cyanides, cyanates and carbonates of the alkali metals having a cyanide content between 0.01 and 3% CN.sup.-.
Nitriding salt baths today are used world wide to improve the wear properties and the fatigue strength of components made of iron and steel. The salt bath process is also finding increasing use for improving the corrosion resistance of the surface of components made of iron and steel, whereby the development of a special salt bath for cooling the parts after nitriding (German OS No. 2,934,113 and related Kunst U.S. Pat. No. 4,292,094 has led to the fact that the nitride compound layer's own corrosion properties can clearly still be improved. The entrie disclosure of Kunst is hereby incorporated by reference and relied upon). Therewith the process of salt bath nitriding has gained great significance also for areas of use where otherwise there is required the use of expensive chromium.
For the purpose of salt bath nitriding baths which contained a high portion of cyanide were originally used. The cyanate needed for the nitriding was produced by aeration, furthermore these baths were operated in titanium crucibles.
The increasing requirement of an ecologically acceptable process have led to the fact that the high cyanide containing salt baths have been replaced by baths practically free of cyanide, whereby by using an organic compound for the regeneration of the bath, the formation of toxic wastes has been eliminated, (German OS 2310815 and related Beyer U.S. Pat. No. 4,019,928, the entire disclosure of which is hereby incorporated by reference and relied upon).
Since cyanide has a strongly reducing action in nitriding baths at temperatures between 550.degree. C. and 650.degree. C., cyanates on the other hand rather have a tendency to release oxygen, the nitriding baths containing only a small amount of cyanide occasionally have the tendency to strongly oxidize the nitride compound layer so that even after the cooling and washing of the component, there remains on the surface a difficult to remove dusty coating. Frequently such coatings are not acceptable for the further use of the nitrided component since they, e.g. with hydraulic aggregates, contaminate the oil and can lead to abrasive wear and tear at susceptible areas, e.g. in the bearings. In order to avoid this type of effect, time consuming cleaning of the components is frequently necessary.
Furthermore rusty red surface coatings can form when components are treated in this type of nitriding bath.
A nitriding bath in the described unacceptable condition--with regard to the surface--is characterized, inter alia, by the fact that a steel foil made from a steel containing 0.05% carbon shows a loss of weight in the order of e.g. some 100 mg/dm.sup.2 when treated for 90 minutes in the salt bath. On the other hand a steel foil of the same composition will show an increase in weight of e.g. 70 mg/mm.sup.2 when treated in a bath which does not show the detrimental condition. However, previously it was not possible to determine beforehand when a bath produced good results and when it produced non-acceptable results in regard to surface cleanliness.
Such baths previously had to be brought into good working condition by overheating the bath for a certain period and thoroughly desludging it so that the components treated afterwards had an acceptable surface quality. However, the bath giving good results remained in that state for only a short time and thereafter the procedure had to be repeated.
Therefore it was the task of the present invention to provide a salt bath for the nitriding of components made of iron and steel consisting essentially of alkali metal cyanide, alkali metal cyanate and alkali metal carbonate (e.g. sodium and potassium cyanides, cyanates and carbonates) having a cyanide content of between 0.01 and 3% CN.sup.- which did not produce surface coatings on the nitrided components even after long operation.